Particles of 100 nm or less in size are generally called nanoparticles, and are just beginning to be used in various fields since they show properties different from those of a general bulk matter composed of the same material. In such a situation, it is becoming important to evaluate the properties of the microparticles.
Dielectrophoresis is known as an evaluation method for microparticle. Dielectrophoresis phenomenon is a phenomenon in which electric polarization is occurred in particles, even if they are charge-free, by applying a strong nonuniform electric field, and the particles are moved due to the attractive force unbalance for polarized positive and negative charges altered by the nonuniform electric field. The intensity of dielectrophoresis is involved by important properties related to physical properties of a particle, such as respective complex dielectric constants of the particle and a medium thereof, and diffusion coefficient. Therefore, by evaluating dielectrophoretic sensitivity of the particle, evaluations for physical properties of the particle and the dispersion medium and for the interaction between the both can be performed.
A typical method for observing the dielectrophoretic intensity of microparticle is direct observation of migrating particles by a microscope. However, the microscopic observation cannot be applied to nanoparticle with a particle size below an optical microscopic observable limit. Therefore, it is proposed to chemically bond a phosphor to the nanoparticle, so that intensity distribution of fluorescent emission can be observed even for a particle smaller than optical resolution. Further, it is known to microscopically observe the process of attracting particles by electrode attractive force and videotape the intensity distribution of fluorescent emission of the particles (refer to, for example, Non-patent Literature 1, 2 or 3), and the dielectrophoretic motion and intensity can be acquired from the video images.    Non-Patent Literature 1: M. Washizu, S. Suzuki, O. Kurosawa, T. Nishizaka, T. Shinohara, “Molecular dielectrophoresis of biopolymers”, IEEE Trans. on Industry Applications, vol. 30, 835-843 (1994)    Non-Patent Literature 2: Nicolas G. Green and Hywel Morgan, “Dielectrophoresis of Submicrometer Latex Spheres. 1. Experimental Results”, J. Phys. Chem. B, vol. 103, 41-50 (1999)    Non-Patent Literature 3: Masao WASHIZU, Tomohisa KAWABATA, Osamu KUROSAWA, Seiichi SUZUKI “Dielectrophoretic handling of bio-molecules and its application to bio-separation”, the IEICE (Institute of Electronics, Information and Communication Engineers) Transactions C, Vol. J83-C, No. 1, pp 1-8, August, 2000